foo.tex

\begin{schemedisplay}
(define engine-tag-L-car (engine-tag-compare car 'L))
(define engine-tag-U-car (engine-tag-compare car 'U))
(define engine-tag-L-cdr (engine-tag-compare cdr 'L))
(define engine-tag-U-cdr (engine-tag-compare cdr 'U))

(define apply-pi
  (lambda (pi v)
    (pmatch v
      (`#f (guard (not (pair? c))) c)
      (`,c (guard (not (pair? c))) c)
      (`(tie-tag ,a ,t) (tie (apply-pi pi a) (apply-pi pi t)))      
      (`(nom-tag __)
       (let loop ((v v) (pi pi))
         (if (null? pi) v (apply-swap (car pi) (loop v (cdr pi))))))
      (`(susp-tag ,pi^ ,x)
       (let ((pi (append pi pi^)))
         (if (null? pi) (x) `(susp-tag ,pi ,x))))
      (`(,a . ,d) (cons (apply-pi pi a) (apply-pi pi d))))))

(define nom
  (lambda (a)
    (list 'nom-tag (symbol->string a))))

(define nom
  (lambda (a)
    (list nom-tag (symbol->string a))))
\end{schemedisplay}